What does the ratio mean for Rapid Plasma Reagin (RPR) test results?

From the Guidelines

The ratio in Rapid Plasma Reagin (RPR) test results represents the degree of dilution at which the patient's serum still produces a positive reaction, indicating the concentration of antibodies present. For example, a ratio of 1:8 means the serum remains reactive when diluted 8 times. Higher ratios (like 1:32 or 1:64) indicate greater amounts of antibody and typically correlate with active syphilis infection, while lower ratios may represent treated infection, early infection, or late-stage disease. These titers are particularly valuable for monitoring treatment response, as effective therapy should result in declining ratios over time. A four-fold decrease in titer (such as from 1:32 to 1:8) generally indicates successful treatment, while stable or increasing titers may suggest treatment failure or reinfection. RPR titers typically decline more rapidly in early syphilis compared to late-stage disease, and some patients may maintain low-positive titers for life despite adequate treatment, a condition known as serofast state 1.

Some key points to consider when interpreting RPR test results include:

• The use of only one type of test is insufficient for diagnosis because false-positive nontreponemal test results occasionally occur secondary to various medical conditions 1.
• Nontreponemal test antibody titers usually correlate with disease activity, and results should be reported quantitatively 1.
• A fourfold change in titer, equivalent to a change of two dilutions (e.g., from 1:16 to 1:4 or from 1:8 to 1:32), usually is considered necessary to demonstrate a clinically significant difference between two nontreponemal test results that were obtained by using the same serologic test 1.
• It is expected that the nontreponemal test will eventually become nonreactive after treatment; however, in some patients, nontreponemal antibodies can persist at a low titer for a long period, sometimes for the remainder of their lives, known as the serofast reaction 1.

In terms of the most recent and highest quality study, the 2020 study by the Clinical Infectious Diseases journal provides the most up-to-date information on the performance characteristics of nontreponemal antibody tests, including the RPR test 1. This study highlights the importance of considering the sensitivity and specificity of these tests, as well as the potential for false-positive results, when interpreting RPR test results.

Overall, the RPR test is a valuable tool for diagnosing and monitoring syphilis, but it should be used in conjunction with other tests and clinical information to ensure accurate diagnosis and treatment 1.

From the Research

RPR Test Results

The Rapid Plasma Reagin (RPR) test is a non-treponemal test used to diagnose syphilis. The test results are reported as a ratio, which indicates the level of antibodies present in the blood.

• A higher ratio indicates a higher level of antibodies, which can suggest a more active or advanced infection 2.
• The ratio is typically reported as a dilution factor, such as 1:64, which means that the blood sample was diluted 64 times before the test was performed 2.

Interpreting RPR Results

The interpretation of RPR results can be complex, and the ratio can vary depending on several factors, including the stage of the infection and the individual's immune response.

• A study found that the RPR titer can change significantly between diagnosis and treatment, with 14.8% of cases showing a ≥4-fold increase or decrease in RPR titer during this interval 3.
• Another study found that the RPR test can be beset with technical and biological limitations, making accurate diagnosis and appropriate treatment problematic 4.

Comparison with Other Tests

The RPR test is often used in combination with other tests, such as the Treponema pallidum particle agglutination assay (TPPA) and the fluorescent treponemal antibody absorption test (FTA-ABS).

• A study found that the RPR test had a sensitivity of 65.1% and a specificity of 98.4%, compared to an ELISA-based test that had a sensitivity of 100% and a specificity of 98.4% 5.
• Another study found that the FTA-ABS test had an agreement of 97.5% with the TPPA test, and 95.9% with the TPPA test 6.

Clinical Significance

The RPR ratio can have significant clinical implications, as it can affect treatment decisions and patient outcomes.

• A study found that a 3-fold difference in RPR results between laboratories can be clinically significant and could result in undertreatment or overtreatment 4.
• Another study found that repeating the RPR test if the day of initial presentation and the day of treatment are different, even when treatment is within a few days after initial presentation, can provide valuable information for clinical practice 3.